This invention relates to the preparation of cellular vinyl chloride polymers. This invention further relates to certain organotin compounds which not only effectively activate the blowing agents employed in preparing cellular vinyl chloride polymers but also impart superior levels of heat stability to the final product.
Cellular vinyl chloride polymers are conventionally prepared by melting a vinyl chloride resin in the presence of a blowing agent which decomposes at the processing temperature to yield a gas. The bubbles of gas evolved are entrapped within the molten resin, thereby forming a cellular structure. The foaming operation is usually performed while the resin is contained in a heated mold or in the heated barrel of an extruder, so that the resin can be simultaneously foamed and shaped into commercially useful articles, including pipe, decorative molding and structural siding. One class of blowing agents often employed in preparing cellular vinyl chloride polymers is the azodicarbonamides, exemplified by azobisformamide ##EQU1## The blowing agent is preferably employed in combination with an activator for the purpose of increasing both the degree and rate of blowing agent decomposition. The resultant larger volume of gas generated is desirable, since it increases the efficiency of the blowing agent, thereby reducing the amount of blowing agent required. Temperatures employed to melt the polymer and decompose the blowing agent are between 150.degree. and 200.degree.C. It therefore becomes necessary to include in the formulation a stabilizer for the purpose of eliminating or at least minimizing the heat-induced discoloration of the vinyl chloride polymer which would otherwise occur at these temperatures. To increase efficiency and reduce costs it would be desirable to employ a single compound which functions effectively as both an activator for the blowing agent and a heat stabilizer.
It is well known that a variety of organotin compounds, particularly dibutyltin derivatives of mercaptocarboxylic acid esters will impart useful levels of heat stability to vinyl chloride polymers. German Pat. Nos. 2,133,372 and 2,047,969 disclose the use of organotin mercaptocarboxylic acid esters in foamed polyvinyl chloride. These compounds stabilize well but do not effectively activate blowing agents such as azobisformamide. Organotin carboxylates such as dibutyltin maleate, dibutyltin dilaurate and dibutyltin maleate-halfesters are disclosed in Japanese Pat. No. 6264/67 as being useful in flexible, i.e., plasticized, polymer foams. Although these organotin compounds activate azodicarbonamides, they are poor thermal stabilizers for the polymer. Thus, it can be seen tha organotin mercaptocarboxylic acid esters impart good thermal stability but poor blowing agent activation, while organotin carboxylates offer good activation, but poor thermal stability with a resultant lack of proper melt viscosity control.
In addition to dimethyltin-, dibutyltin- and dioctyltin-mercaptocarboxylic acid esters, other compounds that are effective heat stabilizers for vinyl chloride polymers but poor blowing agent activators are bis(dialkyltin-monomercaptocarboxylic acid ester) sulfides such as bis(dibutyltin-isooctylmercaptoacetate) sulfide, bis(monalkyltin-dimercaptocarboxylic acid ester) sulfides such as bis(monobutyltin-di-isooctylmercaptoacetate) sulfide, (monoalkyltin-dimercaptocarboxylic acid ester) (dialkyltin-mercaptocarboxylic acid ester) sulfides, e.g., (monobutyltin-di-isooctylmercaptoacetate) (dibutyltin-isooctylmercaptoacetate) sulfide, and monoalkyltin tris-mercaptocarboxylic acid esters, e.g., monobutyltin tris(isooctylmercaptoacetate).
Other organotin compounds have been found to be relatively ineffective with regard to both thermal stability and ability to activate blowing agents. These compounds include monoalkylthiostannoic acids and their anhydrides, e.g., butylthiostannoic anhydride, dialkyltin sulfides, e.g., dibutyltin sulfide, trialkyltin mercaptocarboxylic acid esters, e.g., tributyltin-isooctylmercaptoacetate, and tin-tetramercaptocarboxylic acid esters, such as tin-tetra(isooctylmercaptoacetate).
Organotin carboxylates which are good blowing agent activators but poor thermal stabilizers, are monoalkyltin tris(dicarboxylic acid half-esters) such as monobutyltin tris(dodecyl maleate), dialkyltin dicarboxylic acid compounds such as dibutyltin azelate, and dialkyltin monocarboxylic acid derivatives, e.g., dibutyltin bis(tall oil fatty acid carboxylate) and dibutyltin bis(benzoate).
While it may appear obvious to attain the desired effect by combining one of the foregoing good heat stabilizers with an effective activator for the blowing agent, this approach only slightly improves the overall performance. The mixtures provide adequate thermal stabilization but only a marginal improvement in degree of blowing agent activation.
A number of non-tin-containing stabilizer-activators are currently available for use in rigid cellular polyvinyl chloride formulations. These products are almost exclusively based on compounds of barium, cadmium and lead. A major deficiency of many of these compounds is their relatively high toxicity. In addition, these metal-based compounds have been found to be less effective in static and dynamic thermal stabilization of PVC than many organotin compounds. More importantly, they cause decomposition of the azodicarbonamide blowing agent at so low a temperature that the gas is generated before it can be efficiently utilized, in other words before the polymer is in a completely molten state and therefore capable of entrapping the gas to form the desired cellular structure.
An objective of this invention is to provide foamable vinyl chloride polymer compositions that exhibit good thermal stability and blowing agent activity.
Unexpectedly it has now been found that certain organotin sulfur-containing compounds provide an optimum balance between good blowing agent activation and good thermal stability. Additionally, it has been found that these compounds can be combined with selected oxygen-containing organotin compounds or metal salts of carboxylic acids to provide the desired combination of excellent blowing agent activation and good thermal stability.